The present invention relates to a rotary blower, and more particularly, to a torsion damping mechanism (“isolator”) for reducing audible noise from the blower, and especially from the timing gears.
Although the present invention may be used advantageously on many different types of blowers, regardless of the manner of input drive to the blower, the present invention is especially adapted for use with a Roots-type rotary blower which is driven by an internal combustion engine, also referred to hereinafter as a “periodic” combustion engine because, in the typical internal combustion engine used commercially for on-highway vehicles, the torque output of the engine is not perfectly smooth and constant, but instead, is generated in response to a series of individual, discrete combustion cycles.
It should be understood by those skilled in the art that the present invention is not limited to a Roots-type blower, but could be used just as advantageously in a screw compressor type of device. However, the invention is especially advantageous in a Roots-type blower and will be described in connection therewith. A typical Roots-type blower transfers volumes of air from the inlet port to the outlet port, whereas a screw compressor actually achieves internal compression of the air before delivering it to the outlet port. However, for purposes of the present invention, what is most important is that the blower, or compressor, include a pair of rotors which must be timed in relationship to each other, and therefore, are driven by meshed timing gears. As Is now well known to those skilled in the blower art, the timing gears are potentially subject to conditions such as gear rattle and bounce.
Rotary blowers of the type to which the present invention relates (either Roots-type or screw compressor type) are also referred to as “superchargers” because they are used to effectively supercharge the intake side of the engine. Typically, the input to an engine supercharger is a pulley and belt drive arrangement which is configured and sized such that, at any given engine speed, the amount of air being transferred into the intake manifold is greater than the instantaneous displacement of the engine, thus increasing the air pressure within the intake manifold, and increasing the power density of the engine.
Rotary blowers of either the Roots-type or the screw compressor type are characterized by the potential to generate noise. For example, Roots-type blower noise may be classified as either of two types. The first is solid borne noise caused by rotation of timing gears and rotor shaft bearings subjected to fluctuating loads (the periodic firing pulses of the engine). The second type of noise is fluid borne noise caused by fluid flow characteristics, such as rapid changes in the velocity of the fluid (i.e., the air being transferred by the supercharger). The present invention is concerned primarily with the solid borne noise caused by the meshing of the timing gears. More particularly, the present invention is concerned with torsion damping mechanisms (“isolators”) of the type which can minimize the “bounce” of the timing gears during times of relatively low speed operation, when the blower rotors are not “under load”. The noise which may be produced by the meshed teeth of the timing gears during unloaded (non-supercharging), low-speed operation is also referred to as “gear rattle”.
An example of a prior art torsion damping mechanism for a supercharger is illustrated and described in U.S. Pat. No. 6,253,747, assigned to the assignee of the present invention, and incorporated herein by reference. Such torsion damping mechanisms are also referred to as “isolators” because part of their function is to isolate the timing gears from the speed and torque fluctuations of the input to the supercharger. During the course of the development of a supercharger, including the torsion damping mechanism of the above-incorporated patent, one of the primary developmental concerns has been the durability of the torsion damping mechanism, and therefore, the ultimate service or durability life of the supercharger, in terms of the number of hours of operation, prior to any sort of supercharger component failure.
The torsion damping mechanism of the above-incorporated patent includes a pair of hub members (one attached to the input and the other attached to one of the timing gears), the hub members defining a cylindrical surface. A single torsion spring surrounds, and is closely spaced apart from, the cylindrical surface defined by the hub members. As is now known to those skilled in the art based primarily on the above-incorporated patent, the radial clearance between the cylindrical surface of the hub members and the inside diameter of the generally cylindrical torsion spring is selected to correspond to a predetermined positive travel limit (i.e., greater rotation of the input than of its associated timing gear).
When the torsion damping mechanism of the type to which the present invention relates achieves the predetermined positive travel limit, there is actual surface-to-surface engagement between the inside surface of the coils of the torsion spring and the adjacent cylindrical surfaces of the hub members. In connection with the development of a supercharger embodying the present invention, it has been observed that there has been a wear pattern on the inside surface of the coils of the torsion spring,and that there were iron oxides present on the wear surface of the spring. It has since been determined that the root cause of the wear pattern on the inside surface of the torsion spring is a phenomenon known as “fretting corrosion”. Unfortunately, the configuration of the torsion damping mechanism is such that the torsion spring is “buried” within the mechanism, and any sort of access to the spring during operation is very limited.
Related to the observed fretting corrosion is the known fact that, if the cylindrical surfaces of the hub members wear or corrode to the extent of their diameters being reduced, the “diameter” of the inside surface of the torsion spring will be less than intended, at the positive travel limit of the isolator. Such a decrease in the diameter of the inside surface of the torsion spring will result in changes (an increase) in the level of the stress within the spring, thus typically reducing the life of the spring. A related problem has been observed at the point where one of the coils traverses the axial gap between the hub members, what has been observed is the cutting of a “slot” in the inside surface of the spring where it contacts hub on either side of axial gap. As is well known in the art, the formation of such a slot will result in a stress riser at that location in the spring, further limiting the fatigue life of the isolator spring.